[krbdev.mit.edu #1156] krb5 should be able to use a separate com_err

[Ken Raeburn via RT]

The changes checked in on the trunk add a new configure option
--with-system-et that will cause krb5 to use installed versions of
compile_et and libcom_err.  (And --with-system-ss for mk_cmds and
libss.)  No API change has been made; for "normal" (i.e., non-Mac) UNIX,
it is assumed that the same API can be used as before, calling
add/remove_error_table on et_FOO_error_table; doing otherwise would mean
making the library unsafe for dlopen/dlclose.